1. Field of the Invention
The present invention relates to a method of removing color noise and to an improvement in an image-capturing device that uses this method.
An image-capturing device such as a digital still camera or the like generates image data for one frame by performing processes on an image signal derived from an image sensor as follows: an analog to digital conversion, an analog gain control (AGC), a gamma control, and a signal process such as an image data process which converts the image data into brightness component image data and color difference component image data. A noise component that originates in the image sensor and originates in an intermediate process is included in the image data generated as described.
2. Description of the Related Art
As a conventional method of removing color noise of image data, Japanese Patent Publication No. 2001-189944 discloses a structure configured to separate the image data into brightness component image data and color difference component image data, and change an intensity of a noise reduction in consideration of a spatial correlation between the data. Japanese Patent Publication No. 2004-159176 discloses a structure configured to delete noise of a color component by using a different method of removing the noise on each color of RGB (red, green and blue) without affecting a resolution of the image. Japanese Patent Publication No. 2001-223941 discloses a structure configured to change a degree of the noise reduction according to its importance on a screen. Japanese Patent Publication No. 2001-223941 discloses a structure configured to change a thinning out rate of the color difference component image data according to a state of a photographic subject or a compressibility setting when compressing the data, whereby a volume of information on the brightness component image data side is increased and compression warp noise is reduced. Japanese Patent Publication No. 2002-95009 discloses a structure configured to reduce the noise by shrinking once reproduction image data read out from a recording medium and expanded, and then expanding the data to a previous size.
In the image-capturing device which uses the image sensor such as a CCD or the like, miniaturization of the image-capturing device and an increase in the number of pixels of the image-capturing device have been progressed, and the size of each unit in a light-receiving part of the image sensor has become smaller, so a light-receiving area of the light-receiving part has also become smaller, therefore reduction in a S/N ratio caused by a decrease in an amount of light received has been problematic.
Moreover, there is also reduction in the S/N ratio that originates in enlarging a gain of a signal amplification circuit for raising an image signal level when the photographic subject has low illuminance.
Each of the noises is not a specific pattern, but is a so-called random noise generated in a random manner. A smoothing process is usually performed for removing the random noises.
In addition, when comparing spatial frequencies of the noises between the brightness component and the color difference component (color component), because a frequency of the color noise is low, an effect of noise removal is not achieved without an enlargement of a size of a filter used for the smoothing process.
However, a circuit scale increases along with the enlargement of the filter size for removing the color noise, which results in an increase in cost of LSI. Moreover, power consumption also increases in a process with the filter, and a time inconvenience in a process for removing the color noise is caused.